1. Field of the Invention
This invention relates to a process for preparing an electroluminescent (EL) film. More particularly, it relates to a process for preparing a thin EL film by employing a chemical vapor deposition (CVD) technique which enables the growth of a thin film of high quality on a mass-production basis. The thin film EL device which is produced by using the process of this invention allows multicolored luminescence with high brightness and is, therefore, applicable to a planar display of high quality for an electronic apparatus, a measuring instrument or the terminal component of a computer, and a planar television as well.
2. Description of the Related Art
There has been known a thin film EL device which comprises an electroluminescent film made of a semiconductor compound of Group II-VI elements, such as ZnS, ZnSe, CaS, SrS or SrSe, and doped with a rare earth element, such as Tb, Sm, Eu or Ce, for forming luminescent centers.
A typical thin film EL device which is practically employed comprises an electroluminescent film made of ZnS and doped with Mn forming luminescent centers, two insulating layers of high resistivity between which the electroluminescent film is sandwiched, a transparent electrode on the outside of one of the insulating layers and a back electrode on the outside of the other insulating layer.
In order to achieve a high degree of luminance and reliability, the electroluminescent film is required to contain a relatively high concentration of Mn in the range of 0.1 to 1 at.% and have a high degree of crystallinity. Two processes are practically used for preparing such an electroluminescent film. One of them is the electron-beam evaporation technique which employs sintered pellets of a mixture of ZnS and Mn. The other process is the atomic layer epitaxy (ALE) in which Zn, Mn and S are transported in vapor form to a substrate alternately to form mono layers of atoms one upon another (Japanese Patent Publication No. 35158/1982).
Attempts have been made to form EL devices by molecular beam epitaxy (MBE) (Tomoyoshi, et al.: "Molecular beam epitaxial ZnSe:Mn dc electroluminescent cell with very low threshold voltage", J. Appl. Phys., 52, September 1981, pp. 5797-5799), and by organometallic chemical vapor deposition (OMCVD) (Alan F. Cattell, et al.: "Electroluminescence Fro Films of ZnS: Mn Prepared by Organometallic Chemical Vapor Deposition", IEE TRANSACTION ON ELECTRON DEVICES, vol. ED-30, No. 5, May 1983). Both of these processes are, however, still in the stage of basic research prior to practical use.
Attempts have also been made to employ hydrogen or halogen transport chemical vapor deposition (CVD) for preparing a thin film of a single-crystal compound semiconductor of Group II-VI elements, such as ZnS, for the purpose of making a light-emitting diode or laser. There is, however, not known any case where this CVD technique has been employed to form an electroluminescent film for a thin film EL device by simultaneously doping an element forming luminescent centers which is a kind of impurity.
The EL device is essentially required to have a thin and highly crystalline electroluminescent film in order to realize a high degree of luminance and a reduction in driving voltage. Moreover, it is desirable to employ a film-forming process which makes it possible to dope the film with a high concentration of Mn in a stable and accurately controllable way.
While the electron beam evaporation technique is a process which allows for high film-forming speed and is suitable for use on a mass-production basis, it can form only a film of low crystallinity, since the three-dimensional growth of the nucleus occurs during the initial stage of film formation. The film is, therefore, required to have a large thickness in order to achieve a high degree of luminance, which necessarily brings about an increase in driving voltage. The low crystallinity is also the cause of a reduction in reliability of the film property.
As the film which has been formed by ALE is a product of the two-dimensional growth, it is of high crystallinity and can, therefore, realize a high degree of luminance and a reduction in driving voltage. This process, however, allows a film to grow only very slowly and is, therefore, not suitable for use on a mass-production basis.
While MBE can also form a film of high crystallinity, it has the drawbacks of being unable to form a film having a large area and lacks applicability on a mass-production basis. The OMCVD process is disadvantageous due to the fact that few kinds of organic materials are available for use as a dopant forming luminescent centers.